1. Field of the Invention
The present invention relates to a marker structure for optical alignment of a substrate, a substrate including such a marker structure, an alignment method for aligning to such a marker structure and a lithographic projection apparatus.
2. Description of the Related Art
The term “patterning device” as here employed should be broadly interpreted as referring to device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). An example of such a patterning device is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Another example of a patterning device is a programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam,eaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be seen, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and WO 98/38597 and WO 98/33096. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
Another example of a patterning device is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally<1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792.
In a known manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. It is important to ensure that the overlay juxtaposition) of the various stacked layers is as accurate as possible. For this purpose, a small reference mark is provided at one or more positions on the wafer, thus defining the origin of a coordinate system on the wafer. Using optical and electronic devices in combination with the substrate holder positioning device (referred to hereinafter as “alignment system”), this mark can then be relocated each time a new layer has to be juxtaposed on an existing layer, and can be used as an alignment reference. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens.” However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. Nos. 5,969,441 and 6,262,796.
For a lithographic process, an alignment of the wafer to be processed with the mask pattern on the mask should be as precise as possible for a correct definition of features on the substrate, which features all should have sizes within specified tolerances. To this end, the lithographic projection apparatus includes a wafer alignment module which provides for alignment of the substrate with the mask and mask pattern within a given (specified) tolerance. The wafer alignment system typically performs the alignment based on optical devices. The position of a wafer or a portion of a wafer is determined by measuring an optical response from an optical marker which is illuminated by an optical source: for example, a grating is illuminated by a laser beam, the laser beam diffracts from the grating, and one or more of the diffracted orders are measured by respective sensors, which are typically located on a reference plane. Using the output of the sensors the position of the wafer can be derived (relative to the reference plane).
In the prior art, optical markers include a grating with a periodicity suitable for diffraction of impinging light with a wavelength well within the visible range of the spectrum. A typical periodicity is 16 μm. The grating is typically constructed of lines and trenches. Typically, the line width and trench width are each 8 μm. In order to obtain sufficient diffracted light from the grating and to obtain well-defined diffraction maxima and minima, the grating must encompass a minimal number of lines and intermediate trenches. The size in the direction of the periodic structure is about 750 μm.
The grating may be a phase grating or phase marker which takes into account a phase difference between the phase of rays scattered at the upper surface of the grating and the phase of rays scattered at the lower surface of the grating.
Also, the grating may be an amplitude grating which only takes into account the periodic structure of the grating without any further phase difference relating to an upper or lower level in the grating. Typically, an amplitude grating or amplitude marker is constructed of a periodic structure of first and second elements, which have similar surface levels but different respective reflectance.
Optical markers are used during microelectronic device processing (or IC processing) along the full manufacturing line. During the front end of line (FEOL), markers are used for alignment during manufacturing of transistor structures. At a later stage during the back end of line (BEOL), markers are needed for alignment of metallisation structures, e.g. connect lines, and vias. It is noted that in both cases the integrity of the markers must be sufficient to meet the required accuracy of alignment.
During semiconductor manufacturing processes a wafer is subjected to a plurality of treatments such as annealing, etching, polishing, etc., which may likely cause roughness of a marker (a recessed area in the marker and/or warping of the marker). Such marker roughness causes an alignment error of the image which may contribute to an overlay error in the construction of a semiconductor device. Also, it is conceivable that during the subsequent stages of processing the quality of markers tend to diminish.
A disadvantage of prior art optical markers is that during IC processing it is difficult to control the phase depth of the optical marker. As a result, the intensity of diffracted light under a given diffraction angle may be low, and even close to zero, and accurate measurement of the diffracted beam may be difficult, or even impossible. The phase depth can be defined as the resolved height difference between a top surface of a line and a top surface of a trench in a grating under a given angle of diffraction. If under an angle of diffraction, where (under optimal conditions) a maximum of diffracted intensity is expected, the phase depth equals half a wavelength of the applied radiation, interference between diffracted waves will result in a low or zero intensity.
Control of the phase depth during IC processing may be difficult due to process variations from wafer to wafer, and also across a single wafer.
A further disadvantage of prior art markers results from the dependence of marker properties as a function of the layer(s) below the marker. It is known that due to different optical behaviour of the various layers, as found in semiconductor devices, the contrast of the marker may vary, which results in variations of the diffracted intensity as function of the layer below.
Moreover, it is known that various processing steps may adversely influence the shape of alignment markers. Due to the effect on the shape, the alignment by such modified markers may comprise an error which can result from the fact that the modified shape of the marker changes the generated (pattern of) diffracted beams.
Furthermore, in the prior art, during BEOL processing, optical markers could be detected under capping layers by virtue of the residual structure which was visible at the surface. However, due to application of planarisation processes such as chemical mechanical polishing (CMP), the option to use a residual marker structure for alignment in many cases has become impossible.
In the prior art, markers on a semiconductor substrate that comprise trenches filled with tungsten, are subjected to a CMP process for removing tungsten from the surface and planarising the surface. Due to the CMP process, the tungsten structures are either filled or underfilled. The extent of filling is related to the phase depth of an optical signal generated by the marker, i.e., two discrete phase depth levels exist. One level relates to filled tungsten structures which are shallow and have a small phase depth, the other level relates to underfilled tungsten structures which are relatively deep and have a large phase depth. Small phase depth of the filled markers is undesirable since the alignment error caused by the small phase depth is relatively large. Also, the large phase depth does not guarantee that the alignment error is reduced: the phase depth may be such that extinction of the optical signal results. Furthermore, no control over the phase depth is obtained.
The influence of optical markers on IC processing may lead to undesirable side effects due to the fact that the optical markers are inherently larger than feature sizes in integrated circuits. In the prior art, the minimum feature size of markers is in the order of 1 μm. In current microelectronic devices, the typical minimal feature size is about 100 nm (depending on the device generation). Since the marker usually includes the same material as (part of) the devices, the presence of an additional marker area of a substantial size in the vicinity of a device may have an influence on the local processing rate for that device in a given processing step. For example, a chemical reaction in a reactive ion etching process or a chemical deposition process may be influenced by the presence of a large marker area due to some kinetic constraint, or due to a local deviation of the wafer temperature, etc. A size difference between marker and device feature may thus lead, for example, to modification of a processing step for devices located closely to a marker. Due to the modification of the processing a variation of device characteristics may occur across a die and/or a wafer.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. One of ordinary skill in the art will appreciate that, in the context of such alternative applications, any use of the terms “reticle”, “wafer” or “die” in this text should be considered as being replaced by the more general terms “mask”, “substrate” and “target portion”, respectively.
In the present document, the terms “radiation” and “projection beam” are used to encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range 5-20 nm).